3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: sshd_config.5,v 1.70 2006/08/21 08:14:01 dtucker Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH daemon configuration file
45 .Bl -tag -width Ds -compact
46 .It Pa /etc/ssh/sshd_config
50 reads configuration data from
51 .Pa /etc/ssh/sshd_config
52 (or the file specified with
55 The file contains keyword-argument pairs, one per line.
58 and empty lines are interpreted as comments.
59 Arguments may optionally be enclosed in double quotes
61 in order to represent arguments containing spaces.
64 keywords and their meanings are as follows (note that
65 keywords are case-insensitive and arguments are case-sensitive):
68 Specifies what environment variables sent by the client will be copied into
75 for how to configure the client.
76 Note that environment passing is only supported for protocol 2.
77 Variables are specified by name, which may contain the wildcard characters
81 Multiple environment variables may be separated by whitespace or spread
85 Be warned that some environment variables could be used to bypass restricted
87 For this reason, care should be taken in the use of this directive.
88 The default is not to accept any environment variables.
90 Specifies which address family should be used by
101 This keyword can be followed by a list of group name patterns, separated
103 If specified, login is allowed only for users whose primary
104 group or supplementary group list matches one of the patterns.
105 Only group names are valid; a numerical group ID is not recognized.
106 By default, login is allowed for all groups.
107 The allow/deny directives are processed in the following order:
118 for more information on patterns.
119 .It Cm AllowTcpForwarding
120 Specifies whether TCP forwarding is permitted.
123 Note that disabling TCP forwarding does not improve security unless
124 users are also denied shell access, as they can always install their
127 This keyword can be followed by a list of user name patterns, separated
129 If specified, login is allowed only for user names that
130 match one of the patterns.
131 Only user names are valid; a numerical user ID is not recognized.
132 By default, login is allowed for all users.
133 If the pattern takes the form USER@HOST then USER and HOST
134 are separately checked, restricting logins to particular
135 users from particular hosts.
136 The allow/deny directives are processed in the following order:
147 for more information on patterns.
148 .It Cm AuthorizedKeysFile
149 Specifies the file that contains the public keys that can be used
150 for user authentication.
151 .Cm AuthorizedKeysFile
152 may contain tokens of the form %T which are substituted during connection
154 The following tokens are defined: %% is replaced by a literal '%',
155 %h is replaced by the home directory of the user being authenticated, and
156 %u is replaced by the username of that user.
158 .Cm AuthorizedKeysFile
159 is taken to be an absolute path or one relative to the user's home
162 .Dq .ssh/authorized_keys .
164 In some jurisdictions, sending a warning message before authentication
165 may be relevant for getting legal protection.
166 The contents of the specified file are sent to the remote user before
167 authentication is allowed.
168 This option is only available for protocol version 2.
169 By default, no banner is displayed.
170 .It Cm ChallengeResponseAuthentication
171 Specifies whether challenge-response authentication is allowed.
172 All authentication styles from
178 Specifies the ciphers allowed for protocol version 2.
179 Multiple ciphers must be comma-separated.
180 The supported ciphers are
195 .Bd -literal -offset 3n
196 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
197 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
198 aes192-ctr,aes256-ctr
200 .It Cm ClientAliveCountMax
201 Sets the number of client alive messages (see below) which may be
204 receiving any messages back from the client.
205 If this threshold is reached while client alive messages are being sent,
206 sshd will disconnect the client, terminating the session.
207 It is important to note that the use of client alive messages is very
211 The client alive messages are sent through the encrypted channel
212 and therefore will not be spoofable.
213 The TCP keepalive option enabled by
216 The client alive mechanism is valuable when the client or
217 server depend on knowing when a connection has become inactive.
219 The default value is 3.
221 .Cm ClientAliveInterval
222 (see below) is set to 15, and
223 .Cm ClientAliveCountMax
224 is left at the default, unresponsive SSH clients
225 will be disconnected after approximately 45 seconds.
226 This option applies to protocol version 2 only.
227 .It Cm ClientAliveInterval
228 Sets a timeout interval in seconds after which if no data has been received
231 will send a message through the encrypted
232 channel to request a response from the client.
234 is 0, indicating that these messages will not be sent to the client.
235 This option applies to protocol version 2 only.
237 Specifies whether compression is allowed, or delayed until
238 the user has authenticated successfully.
247 This keyword can be followed by a list of group name patterns, separated
249 Login is disallowed for users whose primary group or supplementary
250 group list matches one of the patterns.
251 Only group names are valid; a numerical group ID is not recognized.
252 By default, login is allowed for all groups.
253 The allow/deny directives are processed in the following order:
264 for more information on patterns.
266 This keyword can be followed by a list of user name patterns, separated
268 Login is disallowed for user names that match one of the patterns.
269 Only user names are valid; a numerical user ID is not recognized.
270 By default, login is allowed for all users.
271 If the pattern takes the form USER@HOST then USER and HOST
272 are separately checked, restricting logins to particular
273 users from particular hosts.
274 The allow/deny directives are processed in the following order:
285 for more information on patterns.
287 Forces the execution of the command specified by
289 ignoring any command supplied by the client.
290 The command is invoked by using the user's login shell with the -c option.
291 This applies to shell, command, or subsystem execution.
292 It is most useful inside a
295 The command originally supplied by the client is available in the
296 .Ev SSH_ORIGINAL_COMMAND
297 environment variable.
299 Specifies whether remote hosts are allowed to connect to ports
300 forwarded for the client.
303 binds remote port forwardings to the loopback address.
304 This prevents other remote hosts from connecting to forwarded ports.
306 can be used to specify that sshd
307 should allow remote port forwardings to bind to non-loopback addresses, thus
308 allowing other hosts to connect.
311 to force remote port forwardings to be available to the local host only,
313 to force remote port forwardings to bind to the wildcard address, or
315 to allow the client to select the address to which the forwarding is bound.
318 .It Cm GSSAPIAuthentication
319 Specifies whether user authentication based on GSSAPI is allowed.
322 Note that this option applies to protocol version 2 only.
323 .It Cm GSSAPIKeyExchange
324 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
325 doesn't rely on ssh keys to verify host identity.
328 Note that this option applies to protocol version 2 only.
329 .It Cm GSSAPICleanupCredentials
330 Specifies whether to automatically destroy the user's credentials cache
334 Note that this option applies to protocol version 2 only.
335 .It Cm GSSAPIStrictAcceptorCheck
336 Determines whether to be strict about the identity of the GSSAPI acceptor
337 a client authenticates against. If
339 then the client must authenticate against the
341 service on the current hostname. If
343 then the client may authenticate against any service key stored in the
344 machine's default store. This facility is provided to assist with operation
345 on multi homed machines.
348 Note that this option applies only to protocol version 2 GSSAPI connections,
351 may only work with recent Kerberos GSSAPI libraries.
352 .It Cm HostbasedAuthentication
353 Specifies whether rhosts or /etc/hosts.equiv authentication together
354 with successful public key client host authentication is allowed
355 (host-based authentication).
356 This option is similar to
357 .Cm RhostsRSAAuthentication
358 and applies to protocol version 2 only.
361 .It Cm HostbasedUsesNameFromPacketOnly
362 Specifies whether or not the server will attempt to perform a reverse
363 name lookup when matching the name in the
369 .Cm HostbasedAuthentication .
374 uses the name supplied by the client rather than
375 attempting to resolve the name from the TCP connection itself.
379 Specifies a file containing a private host key
382 .Pa /etc/ssh/ssh_host_key
383 for protocol version 1, and
384 .Pa /etc/ssh/ssh_host_rsa_key
386 .Pa /etc/ssh/ssh_host_dsa_key
387 for protocol version 2.
390 will refuse to use a file if it is group/world-accessible.
391 It is possible to have multiple host key files.
393 keys are used for version 1 and
397 are used for version 2 of the SSH protocol.
403 files will not be used in
404 .Cm RhostsRSAAuthentication
406 .Cm HostbasedAuthentication .
410 .Pa /etc/shosts.equiv
414 .It Cm IgnoreUserKnownHosts
417 should ignore the user's
418 .Pa ~/.ssh/known_hosts
420 .Cm RhostsRSAAuthentication
422 .Cm HostbasedAuthentication .
425 .It Cm KerberosAuthentication
426 Specifies whether the password provided by the user for
427 .Cm PasswordAuthentication
428 will be validated through the Kerberos KDC.
429 To use this option, the server needs a
430 Kerberos servtab which allows the verification of the KDC's identity.
433 .It Cm KerberosGetAFSToken
434 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
435 an AFS token before accessing the user's home directory.
438 .It Cm KerberosOrLocalPasswd
439 If password authentication through Kerberos fails then
440 the password will be validated via any additional local mechanism
445 .It Cm KerberosTicketCleanup
446 Specifies whether to automatically destroy the user's ticket cache
450 .It Cm KeyRegenerationInterval
451 In protocol version 1, the ephemeral server key is automatically regenerated
452 after this many seconds (if it has been used).
453 The purpose of regeneration is to prevent
454 decrypting captured sessions by later breaking into the machine and
456 The key is never stored anywhere.
457 If the value is 0, the key is never regenerated.
458 The default is 3600 (seconds).
460 Specifies the local addresses
463 The following forms may be used:
465 .Bl -item -offset indent -compact
469 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
474 .Ar host No | Ar IPv4_addr No : Ar port
480 .Ar host No | Ar IPv6_addr Oc : Ar port
487 sshd will listen on the address and all prior
490 The default is to listen on all local addresses.
493 options are permitted.
496 options must precede this option for non-port qualified addresses.
497 .It Cm LoginGraceTime
498 The server disconnects after this time if the user has not
499 successfully logged in.
500 If the value is 0, there is no time limit.
501 The default is 120 seconds.
503 Gives the verbosity level that is used when logging messages from
505 The possible values are:
506 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
508 DEBUG and DEBUG1 are equivalent.
509 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
510 Logging with a DEBUG level violates the privacy of users and is not recommended.
512 Specifies the available MAC (message authentication code) algorithms.
513 The MAC algorithm is used in protocol version 2
514 for data integrity protection.
515 Multiple algorithms must be comma-separated.
517 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
519 Introduces a conditional block.
520 If all of the criteria on the
522 line are satisfied, the keywords on the following lines override those
523 set in the global section of the config file, until either another
525 line or the end of the file.
528 are one or more criteria-pattern pairs.
529 The available criteria are
535 Only a subset of keywords may be used on the lines following a
538 Available keywords are
539 .Cm AllowTcpForwarding ,
543 .Cm X11DisplayOffset ,
546 .Cm X11UseLocalHost .
548 Specifies the maximum number of authentication attempts permitted per
550 Once the number of failures reaches half this value,
551 additional failures are logged.
554 Specifies the maximum number of concurrent unauthenticated connections to the
556 Additional connections will be dropped until authentication succeeds or the
558 expires for a connection.
561 Alternatively, random early drop can be enabled by specifying
562 the three colon separated values
566 will refuse connection attempts with a probability of
569 if there are currently
572 unauthenticated connections.
573 The probability increases linearly and all connection attempts
574 are refused if the number of unauthenticated connections reaches
577 .It Cm PasswordAuthentication
578 Specifies whether password authentication is allowed.
581 .It Cm PermitEmptyPasswords
582 When password authentication is allowed, it specifies whether the
583 server allows login to accounts with empty password strings.
587 Specifies the destinations to which TCP port forwarding is permitted.
588 The forwarding specification must be one of the following forms:
590 .Bl -item -offset indent -compact
604 .Ar \&[ IPv6_addr \&] : port
608 Multiple forwards may be specified by separating them with whitespace.
611 can be used to remove all restrictions and permit any forwarding requests.
612 By default all port forwarding requests are permitted.
613 .It Cm PermitRootLogin
614 Specifies whether root can log in using
618 .Dq without-password ,
619 .Dq forced-commands-only ,
625 If this option is set to
626 .Dq without-password ,
627 password authentication is disabled for root.
629 If this option is set to
630 .Dq forced-commands-only ,
631 root login with public key authentication will be allowed,
634 option has been specified
635 (which may be useful for taking remote backups even if root login is
636 normally not allowed).
637 All other authentication methods are disabled for root.
639 If this option is set to
641 root is not allowed to log in.
645 device forwarding is allowed.
661 .It Cm PermitUserEnvironment
663 .Pa ~/.ssh/environment
667 .Pa ~/.ssh/authorized_keys
672 Enabling environment processing may enable users to bypass access
673 restrictions in some configurations using mechanisms such as
676 Specifies the file that contains the process ID of the
679 .Pa /var/run/sshd.pid .
681 Specifies the port number that
685 Multiple options of this type are permitted.
691 should print the date and time of the last user login when a user logs
700 when a user logs in interactively.
701 (On some systems it is also printed by the shell,
707 Specifies the protocol versions
710 The possible values are
714 Multiple versions must be comma-separated.
717 Note that the order of the protocol list does not indicate preference,
718 because the client selects among multiple protocol versions offered
724 .It Cm PubkeyAuthentication
725 Specifies whether public key authentication is allowed.
728 Note that this option applies to protocol version 2 only.
729 .It Cm RhostsRSAAuthentication
730 Specifies whether rhosts or /etc/hosts.equiv authentication together
731 with successful RSA host authentication is allowed.
734 This option applies to protocol version 1 only.
735 .It Cm RSAAuthentication
736 Specifies whether pure RSA authentication is allowed.
739 This option applies to protocol version 1 only.
741 Defines the number of bits in the ephemeral protocol version 1 server key.
742 The minimum value is 512, and the default is 768.
746 should check file modes and ownership of the
747 user's files and home directory before accepting login.
748 This is normally desirable because novices sometimes accidentally leave their
749 directory or files world-writable.
753 Configures an external subsystem (e.g. file transfer daemon).
754 Arguments should be a subsystem name and a command (with optional arguments)
755 to execute upon subsystem request.
760 file transfer subsystem.
761 By default no subsystems are defined.
762 Note that this option applies to protocol version 2 only.
763 .It Cm SyslogFacility
764 Gives the facility code that is used when logging messages from
766 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
767 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
770 Specifies whether the system should send TCP keepalive messages to the
772 If they are sent, death of the connection or crash of one
773 of the machines will be properly noticed.
774 However, this means that
775 connections will die if the route is down temporarily, and some people
777 On the other hand, if TCP keepalives are not sent,
778 sessions may hang indefinitely on the server, leaving
780 users and consuming server resources.
784 (to send TCP keepalive messages), and the server will notice
785 if the network goes down or the client host crashes.
786 This avoids infinitely hanging sessions.
788 To disable TCP keepalive messages, the value should be set to
793 should look up the remote host name and check that
794 the resolved host name for the remote IP address maps back to the
795 very same IP address.
801 is used for interactive login sessions.
806 is never used for remote command execution.
807 Note also, that if this is enabled,
809 will be disabled because
811 does not know how to handle
815 .Cm UsePrivilegeSeparation
816 is specified, it will be disabled after authentication.
818 Enables the Pluggable Authentication Module interface.
821 this will enable PAM authentication using
822 .Cm ChallengeResponseAuthentication
824 .Cm PasswordAuthentication
825 in addition to PAM account and session module processing for all
826 authentication types.
828 Because PAM challenge-response authentication usually serves an equivalent
829 role to password authentication, you should disable either
830 .Cm PasswordAuthentication
832 .Cm ChallengeResponseAuthentication.
836 is enabled, you will not be able to run
841 .It Cm UsePrivilegeSeparation
844 separates privileges by creating an unprivileged child process
845 to deal with incoming network traffic.
846 After successful authentication, another process will be created that has
847 the privilege of the authenticated user.
848 The goal of privilege separation is to prevent privilege
849 escalation by containing any corruption within the unprivileged processes.
852 .It Cm X11DisplayOffset
853 Specifies the first display number available for
856 This prevents sshd from interfering with real X11 servers.
859 Specifies whether X11 forwarding is permitted.
867 When X11 forwarding is enabled, there may be additional exposure to
868 the server and to client displays if the
870 proxy display is configured to listen on the wildcard address (see
872 below), though this is not the default.
873 Additionally, the authentication spoofing and authentication data
874 verification and substitution occur on the client side.
875 The security risk of using X11 forwarding is that the client's X11
876 display server may be exposed to attack when the SSH client requests
877 forwarding (see the warnings for
881 A system administrator may have a stance in which they want to
882 protect clients that may expose themselves to attack by unwittingly
883 requesting X11 forwarding, which can warrant a
887 Note that disabling X11 forwarding does not prevent users from
888 forwarding X11 traffic, as users can always install their own forwarders.
889 X11 forwarding is automatically disabled if
892 .It Cm X11UseLocalhost
895 should bind the X11 forwarding server to the loopback address or to
896 the wildcard address.
898 sshd binds the forwarding server to the loopback address and sets the
901 environment variable to
903 This prevents remote hosts from connecting to the proxy display.
904 However, some older X11 clients may not function with this
909 to specify that the forwarding server should be bound to the wildcard
918 Specifies the full pathname of the
922 .Pa /usr/X11R6/bin/xauth .
926 command-line arguments and configuration file options that specify time
927 may be expressed using a sequence of the form:
929 .Ar time Op Ar qualifier ,
933 is a positive integer value and
935 is one of the following:
937 .Bl -tag -width Ds -compact -offset indent
952 Each member of the sequence is added together to calculate
953 the total time value.
955 Time format examples:
957 .Bl -tag -width Ds -compact -offset indent
959 600 seconds (10 minutes)
963 1 hour 30 minutes (90 minutes)
967 .It Pa /etc/ssh/sshd_config
968 Contains configuration data for
970 This file should be writable by root only, but it is recommended
971 (though not necessary) that it be world-readable.
976 OpenSSH is a derivative of the original and free
977 ssh 1.2.12 release by Tatu Ylonen.
978 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
979 Theo de Raadt and Dug Song
980 removed many bugs, re-added newer features and
982 Markus Friedl contributed the support for SSH
983 protocol versions 1.5 and 2.0.
984 Niels Provos and Markus Friedl contributed support
985 for privilege separation.
andersk / gssapi-openssh.git / blob